Aluminas and alumina-silicates are well known to be useful as catalysts, adsorbents and catalyst supports. These materials are made by fusing high purity (99+%) aluminum oxide with or without silica (usually as sodium silicate). They may be very porous or non-porous and have a high or low surface area depending upon the use to be made of them. When used to support a catalyst the support may contain any porous, inert material which does not detrimentally influence the catalytic reaction wherein it is employed. An early patent describing a method of making a pelleted alumina catalyst is U.S. Pat. No. 2,499,675.
Representative of the method of making alumina supports is the following description found in U.S. Pat. No. 3,664,970. The particular support therein is said to be useful as a support for a silver catalyst employed in the oxidation of ethylene to ethylene oxide. For this purpose the support material comprises 90 percent or more by weight alpha alumina and 1 to 6 percent by weight silica. This composition is a preferred support material when combined with 0.1 to 0.4 percent by weight baryta.
In the process of making a support, the high-purity aluminum oxide, preferably in the alpha alumina phase, is throughly mixed with temporary and permanent binders. The temporary binders are thermally decomposable organic compounds of moderate to high molecular weight (i.e., molecular weights above about 250) which, on decomposition, produce the pore structure of the support. The permanent binders are inorganic clay-type materials having fusion temperatures below that of the alumina and are responsible for imparting mechanical strength to the finished support. Silica and baryta can also be added in quantity sufficient to obtain a finished support of the desired strength and composition. After thorough dry-mixing, sufficient water is added to the mass to form the mass into a paste-like substance. The catalyst support particles are then formed from the paste by conventional means such as, for example, high pressure extrusion, granulation or other ceramic forming processes. The particles are then dried and are subsequently fired at an elevated temperature which is in the range of 1,200.degree. to 1,600.degree. C.
In the firing step, the temporary binders are thermally decomposed to carbon dioxide and water and are volatilized, leaving voids in the support mass. These voids are the genesis of the pore structure of the finished support. Suitable temporary binders include such materials as the celluloses and substituted celluloses, e.g. cellulose itself, methylcellulose, ethylcellulose, and carboxyethylcellulose, stearates such as organic stearate esters, e.g. methyl or ethyl stearate, waxes and the like. As firing is continued, the temperature reaches the point at which the permanent binder (inorganic clay such as the kaolins or the ball clays) fuses. The catalyst support is then permitted to cool and, during cooling, the permanent binder sets, acting as a cement to bond the catalyst support particles, and thereby impart mechanical strength to the support and ensure maintenance of the pore structure.
Catalyst supports of desired characteristics can be readily produced by the foregoing procedure. Control of pore size, pore size distribution and porosity are readily affected by appropriate adjustment in known manner of the size of the starting alumina particles, and of the particle size and concentration of the temporary and of the permanent binders in the starting mixture. The larger the starting alumina particle size, the greater will be the porosity of the finished catalyst. The more homogenous in size are the alumina particles, the more uniform will be the pore structure. Similarly, increasing the concentration of the temporary binder will also increase the overall porosity of the finished catalyst support.
Earlier patents which describe the making of alumina supports are U.S. Pat. Nos. 2,499,675; 2,950,169 and 3,172,866. Other patents such as U.S. Pat. Nos. 3,222,129; 3,223,483 and 3,226,191 show the preparation of active aluminas. A particular alumina pellet having high mechanical strength is described in U.S. Pat. No. 3,628,914. Methods of making highly porous aluminas are disclosed in U.S. Pat. Nos. 3,804,781; 3,856,708; 3,907,512 and 3,907,982. Alumina carriers having high thermal stability are disclosed in U.S. Pat. No. 3,928,236. Other more recent improve ments in making catalyst carriers are found in U.S. Pat. Nos. 3,987,155; 3,997,476; 4,001,144; 4,022,715; 4,039,481; 4,098,874 and 4,242,233.
High purity alumina is preferred in order to avoid any extraneous elements, e.g. sodium, which might deleteriously affect the catalytic coating. This is especially true for those supports used to make silver catalysts for use in making ethylene oxide. Such high purity supports have been made, but most do not have as good crush strength as do the lower purity supports. Those high strength, high purity supports which have been made have low porosity which is undesirable in supports for use in EO manufacture. Supports used for silver catalysts employed in the oxidation of ethylene to ethylene oxide also are desirably of low surface area, i.e. less than about 1 m.sup.2 /g. It would, therefore, be highly desirable to have high purity, high porosity, low surface area supports of increased strength for use in making silver catalysts for EO manufacture.
A recent patent (U.S. Pat. No. 4,309,313) for a synthetic zeolite is for a composition containing silicon, aluminum, sodium and cesium or thallium oxides. It has utility as a sorbent and in separation and catalytic applications. This composition does not require the high temperatures that are employed in making the compositions of the present invention.
The use of cesium, among other alkali metals of Group I of the Periodic Table of the Elements, as a promoter for silver catalysts employed in the production of ethylene oxide by the partial oxidation of ethylene in the vapor phase is well known. Among relatively recent patents which claim such catalysts are U.S. Pat. Nos. 4,010,155; 4,012,425; 4,123,385; 4,066,575; 4,039,561 and 4,350,616. All of the above patents teach the use of cesium in conjunction with the silver coating on the surfaces of the support whether it be applied prior to, simultaneously with or after the application of the silver coating.
One of the problems with catalysts of the above type is that the conditions of use, e.g. high temperature, cause leaching of the cesium component and/or migration thereof.
The present invention is the discovery that the benefits of cesium addition may be obtained without the disadvantages of the known art by adding cesium compounds to the support during its manufacture. In addition the incorporation of the cesium compound into the support provides it with improved crush strength.